Field report · 665
Understanding stone treatments: heat, irradiation, oiling
A deeper dive into the four major treatment types, what each does, and how to recognize stones that have been through each.
Most commercial gemstones have been treated. Treatment isn't deception — it's the trade's standard practice for making rough material into something beautiful and durable. But every treatment changes something, and knowing what each treatment does helps you understand what you're buying.
Heat treatment
How it works: stones are heated to specific temperatures (often 200-1,800°C, depending on the species) in controlled atmospheres. Heat changes the oxidation state of impurities, which shifts color.
Common applications:
- Sapphire / ruby — heating clarifies the stone (dissolves silk inclusions) and deepens color. Standard for 95%+ of commercial material.
- Tanzanite — heated to convert brown crystal to violet-blue. Universal.
- Aquamarine — heated to remove yellow-green cast and produce pure blue. Common.
- Citrine — most commercial citrine is heated amethyst.
- Topaz (blue varieties) — heated post-irradiation to stabilize color.
Stability: permanent. A heat-treated stone will not revert to its original color under any normal conditions.
Irradiation
How it works: stones are exposed to controlled radiation (typically cobalt-60 gamma rays, x-rays, or electron beams) which creates color centers by displacing atoms in the crystal lattice.
Common applications:
- Blue topaz — colorless topaz irradiated then heated to produce sky blue, Swiss blue, or London blue
- Smoky quartz — most commercial smoky is irradiated rock crystal
- Diamond (treated colors) — some "fancy color" diamonds are irradiated
- Cultured pearls (some) — irradiated to darken
Stability: usually permanent. Some irradiated stones can fade with prolonged sunlight exposure (notably topaz), but most are stable.
Safety note: post-treatment, stones are tested and shipped only after radiation drops to safe ambient levels. The stones you buy do not transfer radiation.
Oiling and fracture-filling
How it works: surface-reaching fractures are filled with substances that have a refractive index close to the stone's, making them invisible to the eye.
Common applications:
- Emerald — almost universal. Cedar oil is traditional; modern polymers and resins also used.
- Ruby (some) — lead glass filling of fractures, sometimes substantial enough to be considered a composite stone
- Quartz (occasional) — for stabilizing rough specimens
Stability: variable. Oil can dry out over years and need re-oiling. Polymer fills are more stable but can yellow or fail at high temperatures. Lead-glass-filled rubies are particularly susceptible to damage from solvents, ultrasonic cleaners, and re-heating during repair.
Dye and surface coatings
How it works: pigment or color-imparting substance is applied to or absorbed into porous stones.
Common applications:
- Agate, jasper, chalcedony — dyed in vivid colors. Very common.
- Lapis lazuli (lower grades) — sometimes dyed to deepen blue
- Turquoise — dyed and impregnated
- Pearls — bleached and dyed
Stability: variable. Some dyes leach with cleaning; some fade with light. Coatings can wear off with handling.
What to ask
For any significant purchase: "What treatments has this stone had, and are they stable?" A reputable seller will know and will tell you.